杨鹏长难句里面的一个简单疑问，求高手回答

Question

however, recent investigations have shown that the concentrations of most mrna's correlate best, not with their synthesis rate, but rather with the equally vari- able rates at which cells degrade the different mrna's in their cytoplasm
近来的调查表明，大意是关于的MRNA浓度并不是与其合成速率相关，而是与（细胞在其细胞质中降解不同的MRNA的）同样却不同的速率最为相关
请问这句话里面at which里面which代表什么？

Answer 1

which永远都用来指代离他最近的东西。所以无论是从规则上还是从语义上，这里指代的都是在它紧前面的vari- able rates。

Answer 2

at which 引导的是定语从句，修饰前面的前先行词 rates. 即. “at the rate of. "